Problem: On the first 4 exams of her physics class, Tiffany got an average score of 86. What does she need on the next exam to have an overall average of 88?
Explanation: Let her score on the next exam be $x$ The sum of all of her scores is then $4 \cdot 86 + x$ The same sum must also be equal to $5 \cdot 88$ Solve: $x = 5 \cdot 88 - 4 \cdot 86 = 96$.